This invention relates to current limiting circuits. More particularly, this invention relates to a bidirectional current limiting circuit that limits current through the circuit in both directions.
The most popular electrical specification for asynchronous serial data communication is the RS-232C standard. The familiar 25-pin "D" connector found on the back of many computers and other devices is used with the standard. Developed long before integrated circuits, the RS-232C standard does not employ 5-volt and ground logic levels. Instead, it uses a positive voltage of between 3 and 15 V to represent a logic 0 and a negative voltage of between -3 and -15 V to represent a logic 1. In addition, RS-232C receivers must be able to withstand signal levels up to +/-25 V to assure compatibility with the older RS-232B standard.
Under the RS-232C standard, two devices connected together for serial communication are referred to as a piece of data terminal equipment (DTE) and a piece of data communications equipment (DCE). The DTE and DCE are connected pin-to-pin. The DTE transmits data on pin 2 (transmitted data) and receives data on pin 3 (received data). The DCE transmits on pin 3 (received data) and receives data on pin 2 (transmitted data). A device's designation as a DTE or DCE is not fixed. In one context a device may be a DTE while it may be a DCE in another.
Because of the pin designations, a problem may arise in serially connecting two devices. Typically the devices are not labeled as DTE and DCE and are connected by cable. If the user mistakenly connects a DTE to another DTE or incorrectly connects the cable to cause both the DTE and DCE to transmit on the same line, then the DTE may be faced with a +/-25 V signal applied to its transmitter circuit. Without protection against the resultant voltage and current overloads, the transmitter or the power supply of the DTE can be damaged.
A second problem is potential damage to the transmitter caused by a short circuit at its output. Such a condition might arise if pin 2 is accidentally shorted to ground. Without some means for limiting the current through the transmitter, the short circuit current may cause it to burn out.
Prior devices for limiting current have relied on the impedance of the output driver circuitry. However, this requires a large power supply to provide sufficient output current. Several RS-232 integrated circuit drivers such as the LT1080 from Linear Technology also use this approach of providing the protection and a large power supply. But such power is not available for battery-powered portable devices such as calculators, where such large power drains could cause memory loss of lock-up.
The present invention provides a solution to the drawbacks of the prior approach by limiting the current that can flow to and from the transmitter without requiring a high impedance output driver. The invention is not restricted to current control in portable devices, of course, but may be used wherever it is desirable to limit current flowing into and out of a device.